A new study suggests that depending on where they are in their menstrual cycles, women have very different reactions towards strange men. At their most fertile, women are more suspicious of men they see as outside of their social or racial group and who they perceive as being physically threatening. Which means that prejudice could, at least partially, be a byproduct of our biology.

Researchers from Michigan State University asked 252 college-age women -- 224 of whom were white and 28 of whom were black -- to complete an online test in which they quickly associated traits like muscularity with photos of men of both races. They also asked a smaller pool of women to complete a similar test, this time asking them to wear a t-shirt that was either red, blue or yellow and then evaluate men who were also assigned one of those three color groups.

The researchers found that when they were at their most fertile, women were increasingly biased against men in a group different from their own -- whether it was according to race, as in the first experiment, or according to t-shirt color, as in the second. Notably, this was only true of women who indicated that they saw the men as physically threatening.

"Studies have shown that people will more readily associate positive things within their own group," said Carlos David Navarrete, Ph.D, an assistant professor of psychology at Michigan State University, who explained that the study was part of a broader effort to understand the various predictors of prejudice. "But the idea that this could shift across the menstrual cycle is pretty novel."

What the small study suggests is that in addition to learned biases, there could be something about women's biology that leads them to protect themselves against men who they believe pose a risk to them, sexually, at the time when they are most "at risk" of getting pregnant. In other words, when women are more likely to conceive a child, they are biologically driven to be more discriminating about who they are going to mate with. Without necessarily realizing it, they protect themselves against men who they believe endanger that choice.

Navarrete says that he was expecting to find that women who were at their most fertile were biased against so-called "out-group," i.e., racially different men they classified as physically imposing, particularly among the white female participants.

"There are deep histories of negative stereotyping of black men of being, perhaps, sexually coercive," he said. "These stereotypes have been around for centuries, so it wasn't too far of a stretch to think we'd see them here."

But he said the fact that women also showed bias in the colored t-shirt experiment was extremely surprising. It indicates that there doesn't need to be a particularly deep cultural history for the prejudice to exist and to fluctuate according to fertility. The women just need to see the men as different.

The study's lead author, Melissa McDonald, told Canada's Times Colonist that the study could be useful in helping get rid of bias.

"A lot of the research in social psychology is focused on biases and prejudice and racism [being] all culturally learned," she told the paper. McDonald added that the research suggests that at least a small part of our bias "is kind of ingrained" and that "the ways by which we could go about ameliorating this bias are going to have to take that into consideration."

But Navarrete chastened that people should not draw any sweeping conclusions from the study.

"It's way too soon to try and say what this really means out there in the real world," he told The Huffington Post. "It's just an interesting study for it's own sake, to try and map out what the predictors of prejudice are, using this bio-behavioral approach."